The present invention relates to the conversion of alkali metal manganate to alkali metal permanganate by means of electrolytic oxidation, and more particularly to the regeneration of an alkali metal permanganate working bath, such as used in processes for the manufacture of printed circuits, whose effectiveness has been diminished by reason of decrease in alkali metal permanganate concentration arising due, at least in part, to reduction of the alkali metal permanganate to alkali metal manganate.
In the manufacture of printed circuits, particularly multi-layer printed circuits, alkaline solutions of alkali metal permanganate (e.g., potassium permanganate, sodium permanganate) have found use in the desmearing and/or etching back and/or cleaning of substrate surfaces, particularly through-hole surfaces, to prepare them for subsequent metallization. See, for example, British Patent Specification No. 1,479,556; U.S. Pat. No. 4,424,380; U.S. Pat. No. 4,515,829; U.S. Pat. No. 4,592,852; U.S. Pat. No. 4,597,988; U.S. Pat. No. 4,601,783; U.S. Pat. No. 4,601,784; U.S. Pat. No. 4,698,124; published PCT Patent Application No. WO 85/05755; Kukanskis, "Improved Smear Removal Process For Multilayer Circuit Boards", IPC Technical Paper No. 435 (October 1982); and F. Tomaivolo, et al., "Alkaline Permanganate Treatment In Etch-Back Processes", Trans. IMF, 1986, 64, 80.
In the desmearing and/or etch-back and/or cleaning process, the mechanism is essentially one of oxidation and, as a consequence, there is corresponding reduction of the manganese species in the working alkali metal permanganate solution (i.e., Mn.sup.+7) to manganese species of lower oxidation state, such as Mn(VI) and Mn(IV), in the form, respectively, of the corresponding alkali metal manganate and manganese dioxide. Indeed, this reduction, particularly to Mn(VI), also can occur spontaneously via disproportionation in the typical highly alkaline permanganate solutions employed in such processes.
Since the effectiveness of the alkaline permanganate working bath in desmearing, etch-back, cleaning and other like functions is substantially dependent upon the concentration of permanganate species in the bath, the reduction and/or disproportionation reactions, occurring during the work or spontaneously, reduce the operating efficiency of the bath. As such, it is necessary to either discard the bath and employ a newly-prepared bath of adequate permanganate concentration or to add new permanganate to the bath to increase permanganate concentration therein. In either case, added expense obviously is involved and even in the latter case, concentrations of reduction products remaining in the bath eventually become so high as to preclude effective replenishment just by adding additional permanganate.
In recognition of these problems, the art has proposed regenerating such reduced permanganate concentration working baths, and/or prolonging the useful operating life of such baths, by the addition thereto of a chemical oxidizing agent capable, at least in theory, of oxidizing the reduction products (e.g., manganate, manganese dioxide) back to the permanganate species. See in this regard U.S. Pat. Nos. 4,592,852; 4,629,636; and 4,698,124. The obvious problem with procedures of this type is the need to add often expensive chemical ingredients to the bath, often in fairly large amounts, thereby increasing the cost of operation and running risk that the chemicals or their by-products may eventually lead to problems in bath operation or eventually preclude further regeneration in this manner.
As is discussed in further detail hereinafter, the present invention avoids these problems by utilization of electrolytic oxidation to convert reduced manganese species, such as manganate, to the desired permanganate species, thereby regenerating the permanganate working bath. In addition, the invention provides a means, wholly apart from the context of regeneration of working permanganate baths, for producing permanganate from manganate.
As such, additional background art of interest includes Okabe, et al., U.S. Pat. No. 3,986,941, who describe the preparation of an alkali metal permanganate in high purity by electrolytic oxidation of a slurry of manganese dioxide or alkali metal manganate in caustic alkali having a concentration in the range of 10 to 25 percent by weight at temperatures higher than about 60.degree. C., which conditions are stated to be critical to successful operation of the process; Mazuchelli, et al., U.S. Pat. No. 3,293,160, who describe the electrolytic manufacture of manganates and/or permanganates utilizing a sacrificial anode comprised of manganese metal deposited on a conductive core; and Innes, et al., "Plating And Surface Finishing", November 1978, pp. 36-40, who describe the electrolytic regeneration of chromic acid baths employed to etch ABS plastics using a metal tin-lead anode and a cathode comprising a copper electrode immersed in a 4.6 N sulfuric acid solution contained in a porous ceramic container.